Some steamships now have theoretical permission to use the larger waterways (e.g. a Channel paddle ship may use a large river or large ship canal, a Clyde cargo ship also a medium river or standard ship canal), but in practice cannot because of weight limits: both large and medium rivers have a weight limit of 1000 tons, and a ship canal a weight limit of 999 tons, but the ships have weights over 3000 tons. For consistency, either the way weight limits should be increased, or the theoretical permission withdrawn.
Best wishes,
Matthew
Hadn't spotted that - weight limits now increased. Thank you for pointing that out.